This post is full of spoilers about the 2003 reimagined series.

Throughout the series, a common trope is finding out that a character has secretly been a Cylon the entire time. Most of the time, these secret Cylons are not even aware that they are Cylons. They are under the impression that they are actually a human.

I would argue that every character we meet (except for the few "angels") are actually Cylons. We literally never meet a real human in this series.

The rough chain of events were as follows:

1. Humans created Cylons
2. Humans/Cylons did not get along
3. Cylons moved away
4. Cylons developed their own culture in isolation
5. Cylons forgot that they were robots and began to view themselves as true biological human beings
6. Cylons (who believed themselves to actually be humans) created Cylons

And thus becomes a cycle. In each iteration of this loop, these "humans" create Cylons, and from their own perspective, they are inventing the concept of Cylons for the first time. But in reality, each of these loops is just the previous generation of Cylons creating the next generation of Cylons.

We spend the entire series following a group of individuals who we assume are humans. And these individuals themselves even assume they are humans as well. But unbeknownst to them, they are actually Cylons who have just spent so long in isolation that they started to believe they were the true human race.

But true humans are a thing of the past. They are likely living in peace in some distant galaxy after the first generation of Cylons had fled. We never see a real human.